The invention is directed to monitoring the quality of a signal received over an optical network, and in particular, to an eye quality monitor for a data regenerator.
Optical signals suffer between the transmitter and receiver from two principally different groups of degradations that will cause bit errors: noise and distortion. The causes, behavior and remedies for these groups are different. The primary sources of noise are the receiver noise (i.e. shot, thermal), optical bandwidth, interferometric cross-talk, laser noise, reflections, etc.
Distortion is defined as any inaccurate replication of a signal transmitted over a communication link, and could be referred to any network element (NE) along the link. The pulse distortion in a fiber optic system may, for example, be caused by some parts of the light pulses following longer paths (modes) than other parts. The primary sources for distortion are chromatic dispersion, inter-symbol interference, non-linearity of the elements and transmission medium, receiver frequency response, etc. In addition, in amplified wavelength division multiplexed (WDM) systems, the transmission characteristics vary from one channel to another due to the non-flat gain and noise profile of erbium-doped fiber amplifiers (EDFAs). Distortion can be measured by assessing the difference between the wave shape of the original signal and that of the signal at the network element of interest, after it has traversed the transmission link.
In the last decade, the transmission rates of data signals have increased very rapidly, along with the demand for receivers with high sensitivity. For high rate transmission, such as rates over 10 Gb/s, the signal corruption introduced by the transmission channel is a critical parameter. Numerous methods have been proposed to overcome the difficult problem of measuring or estimating the signal quality. However, they are often based on the same basic principles, or use the same equipment, and differ mainly in how the primary measurement is analyzed.
Some signal quality monitoring methods require detection, synchronization, demultiplexing, and then some analysis of the sampled signal. Even in the best cases, the results are uncertain and it is difficult to ascertain if the monitoring method mimics the behavior of the final receiver.
Other methods are based on power monitoring of each channel. As these are averaging methods, they do not sense the pulse distortion. Moreover, the precision required in the spectral measurement is prohibitive in itself.
The extent of signal degradations may be directly measured using an eye closure diagram, which is the graphic pattern produced on an oscilloscope when the detected signal is applied to the vertical input of an oscilloscope and is synchronized with the instrument time base. Changes in the eye opening indicate intersymbol interference, amplitude irregularities, or timing problems. For a binary signal, the eye diagram has a single eye, which is open or closed to an extent determined by the signal degradation. An open eye pattern is desired.
Currently, the time base of the oscilloscope is triggered using a clock signal necessarily extracted from the transmission in order to capture the eye diagram. Consequently, a network provider cannot measure the quality of the optical signal without a clock extract circuit. This prior art method also fails to separate the eye closure due to distortion from that due to noise.
A receiver regenerates the signal presented to it by interpreting the levels of the received signal according to a decision level, defined also as threshold level, or as a slicing level. Generally, binary data regenerators are provided with a fixed threshold level selected so as to yield the best error rate at a predetermined signal power level.
When the extent of signal degradations must be assessed, the current way of doing so is to recover the clock at the site of the regenerator, thus destroying bit rate transparency. Such methods not only increase the complexity, and hence the cost of the equipment at the regenerator site, but also are dependent on the rate of the signal traveling along the link.
For example, U.S. Pat. No. 4,823,360 (Tremblay et al., issued Apr. 18, 1989 and assigned to Northern Telecom Limited) discloses a device for measuring quality of a signal travelling along an optical fiber, using eye closure. The device described in this U.S. patent provides the receiver with three threshold levels for recovering data. Two of the thresholds V1 and V2 are obtained by setting them above and below the center of the eye for a preset error rate, and the third threshold is provided in a selected relationship to the other two. If V1 and V2 are set for equal xe2x80x98errorxe2x80x99 rates, then the central circuit operates near the middle of the eye and hence with a negligible true error rate.
The technique described in the ""360 patent is based on recovering the signal clock, i.e. implies knowing the rate of the channel. In addition, the measurement does not give an indication as to the separate contribution of the noise and the distortion.
However, not all regenerators installed in a transmission link are provided with means for recovering the clock. From this point of view, regenerators may be classified as 1R, that only regenerate the signal, 2R that regenerate and reshape the signal, and 3R, that regenerate, reshape and retime the signal. 2R regenerators are used, for example, at sites where the transmission signal needs to be converted from one wavelength (e.g. 1.3 xcexcm short reach) to another (e.g. a wavelength on the ITU grid suitable for dense WDM).
There is a need to measure the quality of an optical signal, while maintaining bit rate transparency, so that the measurements may be applied to signals of various rates.
It is an object of this invention to provide a method and apparatus for measuring the quality of a signal of a communication network, that alleviate totally or in part the drawbacks of the current methods and apparatuses.
It is another object of the invention to provide a method and apparatus for measuring the quality of a signal in a communication network which are bit rate transparent.
Still another object of the invention is to obtain a measurement of the quality of the optical signal, which can be used to assign the cause of errors to noise or to distortion.
According to one aspect of the invention, there is provided a method of monitoring the quality of an optical signal at a regenerator site, without performing a clock recovery operation, comprising, generating a threshold voltage VTH to take a plurality of values according to a pattern, applying the threshold voltage VTH on a first input of a slicer, and applying an input voltage Vin on a second input of the slicer to obtain a slicer output voltage VS, the input voltage Vin being an electrical equivalent of the optical signal, for the plurality of threshold voltages, obtaining a corresponding plurality of associated parameters; and processing all the associate parameters as a function of all the threshold voltages to simulate an eye diagram of the optical signal.
According to a further aspect of the invention, there is provided an eye quality monitor for a data regenerator for providing a simulated eye diagram of an optical signal without performing a clock recovery operation, comprising a pattern generator for generating a threshold voltage VTH according to a pattern, a slicer, for receiving a the threshold voltage VTH on a first input and an input voltage Vin on a second input, and generating a slicer output voltage VS, the input voltage V being an electrical equivalent of the optical signal, an average detector for providing an associated average voltage VAV of the slicer output voltage VS corresponding to the a threshold voltage VTH, and means for processing the average voltages VAV, to simulate an eye diagram of the optical signal.
An important advantage of the present invention is that the quality of the signal can be measured with no need to recover the bit rate of the signal, which is very important in the case of 2R receivers, where the clock is not available. This results in important savings on equipment at such receivers.
Another advantage of the invention is that it can be applied to various signal rates and is independent of the technology used. Thus, the method can be applied to the existing SONET technology, and also to emerging optical transport networks (OTN).